In recent years, fuel cells have attracted attention as clean, convenient energy sources. A fuel cell is a device designed to obtain electric energy from hydrogen obtained by reforming of raw fuels such as natural gas, methanol and gasified coal gas, and oxygen in the air. In principle, the fuel cell works oppositely to the electrolysis of water; it receives hydrogen and oxygen to form water, yielding direct-current power.
A fuel cell has desirable properties; it has high generation efficiency, it can harness electricity and heat at the same time, it ensures satisfactory preservation of the environment due to reduced amounts of air pollutants and limited noises, and it can make the rate of energy recovery higher than could be achieved with internal combustion engines. Depending on the type of the electrolyte used, the fuel cell is typically broken down into a phosphoric acid type, a molten carbonate type, a solid oxide type, and a solid polymer type.
Among others, solid polymer fuel cells using ion exchange resin membranes as electrolytes, because of generating high output densities on low-temperature works of the order of 60 to 80° C., are now under development in the form of relatively small generators used in buildings or plants, small generators for residential use purposes, power sources for zero-emission vehicles, and any other dispersion type power sources.
As shown in FIG. 1, a solid polymer fuel cell incorporates an ion exchange resin membrane 1 comprising a solid polymer as a solid electrolyte that is a proton conductor, and has an overall solid structure wherein gas diffusion electrodes 2 and 3 are joined to both sides of the membrane 1. With hydrogen fed to the side of anode 3 and oxygen or air to the side of cathode 2, the hydrogen is oxidized on the side of anode 3 to generate protons and electrons. The protons migrate through the membrane 1 with water molecules to the opposite side of cathode 2 where they are used for reduction of oxygen together with electrons fed from an external circuit 4 (load 5) to create water.
Solid polymer fuel cells, now available, are of various types. In a common type, a membrane-electrode combination, as shown in FIG. 1, is closely laminated on both sides with conductive separators 6 and 7 having some functions of polar chamber separation and gas feed passage via sealing gaskets 8 and 9, as shown in FIG. 2. A solid polymer fuel cell is used in the form of a stack wherein a number of such single cell units are stacked together.
A separator is a basic component material for a module having, in addition to the function of polar chamber separation, a function of forming gas flow passages or manifolds, a cooling function and a function of distributing materials uniformly. More specifically, a solid polymer fuel cell separator is required to have functions of ensuring a flow passage for a reaction gas entering in the fuel cell, transmitting electric energy generated at the fuel cell unit to the outside, and dissipating heat generated at the fuel cell unit to the outside.
For separator materials, carbon plates having acid resistance as well as satisfactory electrical conductivity and gas impermeability are often used because the surfaces of the ion exchange membrane possess strong acidity, and are provided on the surfaces with gas flow passage-forming grooves. However, costly machining is needed for grooves by cutting or the like; a problem with the carbon plate separator is that the separator cost accounts of a substantial portion of the total cost of a solid polymer fuel cell. Thus, various approaches have been proposed for improvements in separators.
JP-A 5-109415 proposes that a pure copper plate with gas feed grooves or manifolds formed by etching therein is used as a gas separator for a fuel cell incorporating an ion exchange membrane. However, problems with the metal plate are that its weight is heavy, costs for forming flow passages are high, no sufficient output is obtainable because of increased contact resistance between electrodes and the separator, etc.
JP-A's 11-126622 and 11-126621 proposes a separator for a low-temperature type fuel cell wherein a plating layer comprising a metal or a metal alloy with carbonaceous particles dispersed therein is formed on the surface of a stainless steel substrate, and the carbonaceous particles are exposed on the surface of the plating layer. However, problems with this separator are again that its weight is heavy, costs for machining flow passages are high, and there is concern about long-term reliability due to possible peeling of the plating layer.
WO97/02612 proposes a separator for a solid polymer electrolyte fuel cell, which comprises a carbon composite material wherein expanded graphite powders having a specific particle diameter are dispersed through a thermoplastic or thermosetting resin or its fired product. However, the expanded graphite is poor in handleability, and so the cost of its mass production becomes unavoidably high. When the expanded graphite is extruded through a common extruder into a carbon composite material, it cannot be packed therein at a high density because it is prone to cleave, failing to ensure any sufficient electrical conductivity necessary for a solid polymer fuel cell separator. When a separator comprising a carbon composite material is produced by compression molding, productivity becomes worse with an increased cost, because grooves for feeding oxidizing agent gas and fuel gas have to be formed by machining.
JP-A 2000-243409 proposes a solid polymer fuel cell separator formed of a thermoset carbon-resin material comprising carbon powders and a thermosetting resin. However, the separator formed of the thermoset carbon-resin material costs much. Another problem is that when thinned for the purpose of weight reductions, its strength cannot meet the mechanical properties demanded for fuel cells.
A solid polymer fuel cell separator is required to have high electrical conductivity and low gas permeability as well as improved mechanical properties, heat resistance, chemical resistance and dimensional stability. In consideration of cost reductions, a separator production process for which costly machining such as cutting is not necessary is also desired. For this reason, it has been proposed to make use of composite materials comprising resins and conductive carbon substances, as already described, in anticipation of formation of grooves, manifolds, etc. by pressing, extrusion molding, etc. upon separator formation.
However, the resin/conductive carbon substance composite materials are prone to increase in electrical resistance although depending on resin components. It is thus desired to lower electrical resistance by decreasing the proportion of the resin component and form the separator in the form of a thin layer; however, this renders moldability or formability, fine machinability and mechanical strength low.